This invention relates to a start capacitor assembly for an AC motor, and more specifically relates to an enclosure for a capacitor and a thermistor electrically connected in series with the capacitor. The enclosure, including a case and a cap, is fabricated from a relatively low temperature plastic with a high-temperature plastic core for holding the thermistor.
A single-phase permanent split-capacitor (PSC) induction motor is often used to drive a compressor for an air conditioning system. A PSC motor has two windings, a main winding and an auxiliary start winding. A capacitor, whose value is chosen as a compromise between start and run performance, is placed in series with the start winding. This capacitor also affects phase shift of the run windings and thus increases both starting and run torque, though it is not optimized for either. When power is applied, both windings contribute, aided by the phase shift of the capacitor, to the necessary torque to cause initial torque and acceleration. As the motor achieves operating speed, both windings continue to contribute to the motor""s torque although the run winding now contributes to a far greater degree due to its lower impedance.
A conventional starting device for a PSC motor includes a capacitor in series with a positive temperature coefficient (PTC) resistor or thermistor. The resistance of this thermistor increases with rising temperature. The starting device, comprising a capacitor which is optimized for providing starting torque and a thermistor, is connected in parallel with the existing capacitor of the PSC motor. When power is connected to the motor, the ac current is connected to the windings on the motor through both the existing and the extra start capacitor (through the thermistor which has a low resistance when cold). As current flows to the windings through the capacitor and the thermistor, the thermistor heats up, and its resistance increases. The increased resistance of the heated thermistor reduces the current through the start capacitor. At some point, the temperature and current through the thermistor reach steady state, with only a very small current connected through the start capacitor to maintain the temperature of the thermistor.
The starting device of a capacitor and thermistor is generally packaged as a single unit with two wire leads for connection to the start winding of the motor. Because the thermistor remains hot during the normal operation of the motor, the enclosure for the capacitor and thermistor must be able to withstand the heat generated by the thermistor.
U.S. Pat. No. 4,118,753 discloses an enclosure for housing a starting device including a capacitor and thermistor. The enclosure includes an elongated cylindrical case which is closed at one end and closed by a cap at the opposite end. The enclosure has two transverse intermediate walls of heat insulating material that divide the enclosure into three sections along the longitudinal dimension of the enclosure. Each intermediate wall has a spring contact that extends into the center section between the intermediate walls. The thermistor is located between and engages the spring contacts. The thermistor is disk shaped with opposite sides being the electrical contacts for the thermistor. The thermistor is oriented with its axis extending parallel to the longitudinal dimension of the case. In order to hold the thermistor between the intermediate walls in engagement with the contact springs, a combination of standoffs and rivets are employed to maintain the spacing between the intermediate walls against the force exerted by the spring contacts. Thus the center section with the thermistor is separately fabricated with the standoffs and rivets prior to insertion into the case. The case within the center section has a vent hole for dissipation of heat. The vent hole will also allow access by moisture, dust, insects, and the like to the spring contacts and the thermistor. There is no specific disclosure concerning the plastic used for the enclosure other than the material of the case and the cap are the same material, and the intermediate walls are heat insulative.
Another prior art starting device is shown in FIGS. 1 and 2. The starting device 1 includes a plastic encased capacitor 2 topped by a plastic enclosure 3. The capacitor 2 has an outer cylindrical shell 14 and electric terminals 15 and 16. The plastic enclosure 3 comprises an outer cylindrical shell 4, an internal support structure 5, and a cap 6. A thermistor 7 is supported within recess 8 of the internal support structure 5 of the enclosure 3. The thermistor 7 is sandwiched between contacts 9 and 10 which are in turn connected to wires 11 and 12 respectively. The contacts 9 and 10 have spring elements and are held in contact with the thermistor 7 by protrusions 19 extending from the inside of cap 6. Thus the springs in the contacts 9 and 10 tend to force the cap 6 loose from the shell 4 of the enclosure 3. Therefore, during assembly, the cap 6 must be held against the spring force onto the shell 4 of the enclosure 3 during welding or gluing of the cap 6 to the shell 4. Wire 11 is connected to terminal 16 of capacitor 2, and wire 13 is connected to terminal 15 of capacitor 2. The enclosure 3, including the cylindrical shell 4, the internal support structure 5, and the cap 6, is made of high temperature plastic to withstand the heat generated by the thermistor 7. On the other hand the outer plastic shell 14 of the capacitor 2 is made of a low temperature plastic based on cost considerations. Because the shell 14 of the capacitor 2 and the enclosure 3 are made of different plastic materials, the capacitor shell 14 and the enclosure shell 4 cannot be reliably connected to each other by welding or gluing. Consequently, for the prior art starting device 1, the shell 14 of the capacitor 2 and the shell 4 of the enclosure 3 for the thermistor 7 are mechanically connected by a series of metal clips 17. The metal clips 17 engage the support structure 5 of the enclosure 3 by means of a tab 18. The clips 17 then frictionally engage the capacitor shell 14 by means of a series of sharp barbs along the length of each clip 17 which dig into the shell 14 and resist separation of the enclosure 3 from the capacitor 2. Because the attachment is mechanical, any leakage by the capacitor around its terminals will also leak out of the enclosure 3. In addition, because the enclosure fits over the shell 14 of the capacitor 2, a protruding flange 19 is created which is aesthetically objectionable and which makes the resulting assembly larger in diameter than the capacitor shell alone would be without the cap 6.
The present invention solves the above problems by providing an enclosure for a starting device comprising an outer case and cap both constructed of a low cost, low temperature plastic for enclosing the capacitor and the thermistor of the starting device. The case is cylindrical to accommodate the cylindrical shape of a conventional metal can starting capacitor. The case has a closed end and an open end to which the cap is fitted and substantially sealed. Because the cap and the case are made of the same low temperature plastic, attaching the cap to the case can be reliably accomplished by welding or gluing.
The thermistor is supported within the case by means of a core assembly. The core assembly consists of a high temperature molded plastic core which includes at least one chamber for accommodating the thermistor, and first and second electric contacts for engaging the terminals of the thermistor. The contacts are frictionally fitted on the sides of the chamber. The chamber in the molded plastic core is dimensioned so that when the disk shaped thermistor is inserted into the chamber, the thermistor is oriented with it thickness dimension (and axis) extending in the transverse dimension of the case, and the thermistor""s diameter is oriented in the longitudinal direction of the case. Because the thermistor is oriented in the longitudinal direction of the case, the contact pressure between the contacts in the chamber and the thermistor is contained by the sides of the chamber. No pressure from the electric contacts is exerted on the case or cap of the enclosure.
Therefore, it is an object of the present invention to provide an enclosure for a starting device including a capacitor and thermistor connected in series.
More particularly it is an object of the present invention to provide a starting device enclosure with an outside case and cap that is constructed primarily of a low cost, low temperature plastic that may be glued or welded together.
It is further an object of the present invention to provide an enclosure with a core assembly for holding the thermistor that does not require any fasteners or the like to hold the thermistor in place.
It is also an object of the present invention to provide an enclosure with a core assembly for holding the thermistor in which the electric contact with the thermistor does not exert any pressure on the outside case or cap.